Field of the Invention
The field of the invention is that of Doppler anemometry that requires a knowledge of either the direction of the wind speed, or that of the speed of the carrier of the anemometer when the latter is mounted on a vehicle. More specifically, the field of application is aeronautics and, even more specifically, that of helicopters.
Description of the Related Art
The current anemometric systems of helicopters are, just like those of aeroplanes, based on measurements of total pressure by Pitot probes and of static pressure. These systems are, however, ill suited to the needs of helicopters because they do not make it possible to cover all of their flight domain. On the one hand, the air speed measurement is unavailable at low speed, up to approximately 35 knots, because of the disturbances generated by the flux from the rotor. On the other hand, the anemo-barometric systems of helicopters do not provide the three components of the speed vector but essentially its component on the longitudinal axis of the carrier.
The Doppler “lidar”, LiDAR standing for “Light Detection And Ranging”, partly resolves these various inadequacies. Indeed, it makes it possible to perform a remote speed measurement, from outside the flux of the rotor of the helicopter, without having to use a nose probe. The use of a plurality of laser beams or of a beam scanning system makes it possible to access the three components of the air speed vector throughout the whole flight domain of the carrier.
The signal from the simple homodyne Doppler lidar, which corresponds to the beat between a wave backscattered by the atmospheric particles and a copy of the transmitted wave, gives access only to the absolute value of the projection of the speed vector along the measurement axis and the information concerning the sign of the speed is then lost. In practice, since the heterodyne signal is real, its spectrum, obtained as the square of the modulus of its Fourier transform, is even in frequency and there is no way to determine whether the measured Doppler shift is positive or negative.
On an aeroplane, this does not pose any particular problem in as much as it is possible to find configurations of the sight axes for which the sign of the speed is always the same throughout the flight domain. On the other hand, given the capabilities of the helicopter to move in all directions, but also the need to perform a measurement of the modulus and of the orientation of the surface wind, it is vitally important to have a signed speed measurement.
There are various solutions for determining the sign of the speed. The use of an acoustic-optic modulator or “AOM” makes it possible to shift the frequency of the transmitted optical wave or, in an equivalent manner, of the local oscillator, such that, at zero speed, the beat between the backscattered wave and the local oscillator is no longer at zero frequency but shifted by a frequency fAOM. For example, for a lidar operating in the near infrared at the wavelength of 1.55 μm, it is common practice to choose an AOM that provides a shift of 40 MHz. The frequency range covered by the frequencies lying between 0 Hz and 40 MHz then corresponds to the negative speeds of around −30 m/s to 0 m/s and the frequencies beyond the frequency fAOM correspond to the positive speeds.
The use of an acousto-optic modulator does however present a number of drawbacks:                Reliability: the acousto-optic modulator is a fragile component, notably in a severe thermal and vibratory environment and is not therefore suited to the aeronautical environment;        Cost: the cost of the acousto-optic modulator is high relative to the cost of the optical architecture as a whole;        Increased frequency range. For a symmetrical speed range, the Doppler frequency range to be analyzed is doubled, the computation power needed at the processing level is commensurately increased.        
FIG. 1 represents an optical architecture of “CW”, or “Continuous Wave”, type with acousto-optic modulator.
A laser source 10 transmits an optical wave of frequency νL or of wavelength λL. The latter is shifted in frequency by means of the modulator 11, passes through the optical splitter 12 and then is focused in the atmosphere using a transmission-reception telescope 13. The wave backscattered by the particles P naturally present in the air is shifted in frequency by Doppler effect by a quantity fD carrying speed information V on the axis of the laser beam. The conventional relationship fD=2·V/λL applies or:V=fD·λ/2
The beat between this backscattered wave and the local oscillator produced by the interferometer 14 is detected by the photodetector 15 and produces an electrical signal of frequency fMAO+fD. A spectral analysis by processing means 16 which can, for example, be an averaged periodogram, makes it possible to bring out the noise signal and extract the frequency information.
A different device makes it possible to access the sign of the speed without having to use an acousto-optic modulator. It is represented in FIG. 2. The principle of operation consists in modulating the frequency of a laser source 20 by means of a frequency ramp generator 21. This device makes it possible to measure both the speed V=fD·λ/2 and the distance D separating the target from the anemometer.
α and −α are used to denote the slopes of the frequency ramps of the ramp generator. The device comprises a transmission-reception channel comprising a splitter 22, an amplifier 23, a circulator 24 and a transmission-reception telescope 25. The device also comprises a reference channel comprising a first delay line 26, a second splitter 27, a third splitter 28, a second delay line 29, a first interferometer 30 and a first detector 31. Finally, the device comprises a measurement channel comprising a second interferometer 32 and a second detector 33. By separately processing the signals from frequency ramps of slope +α and −α, the measurement channel is used to respectively measure the frequencies
      f    +    =                                                  f            D                    -                                    2              ⁢              α              ⁢                                                          ⁢              D                        c                                      ⁢                          ⁢      and      ⁢                          ⁢              f        -              =                                                f            D                    +                                    2              ⁢              α              ⁢                                                          ⁢              D                        c                                      .      
By way of example, if the slope α is 6 MHz/μs, if the distance D is 25 m, then
            2.      ⁢              α        .        D              c    =      1    ⁢                  ⁢          MHz      .      If the speed is positive, then the Doppler shift is +5 MHz, the frequency f+ is 4 MHz and the frequency f− is 6 MHz. Conversely, if the speed is negative, then the Doppler shift is −5 MHz, the frequency f+ is 6 MHz and the frequency f− is 4 MHz. Thus, it is possible to retrieve, by comparing the difference between the frequencies f+ and f−, not only the value of the speed, but its sign. The difference between the two frequencies is representative of the distance to the object.
This type of device does however present a number of drawbacks. Among others, it requires the use of specific laser sources, with a waveform that is well controlled and monitored by means of an additional detection channel, thus increasing the number of components needed.